The present invention relates to mineral-insulated cable terminations, and more particularly to a terminal design for such a cable which provides a high-temperature, electrically insulating, hermetically sealed termination for protecting the mineral insulation from loss or deterioration in moist, high temperature, physically demanding environments.
Mineral-insulated cables as presently used in the art are high-temperature cable structures typically comprising one or more wire conductors contained within a bendable metal sheath. A mineral filler is disposed in the sheath to maintain separation and electrical isolation between the sheath and the conductors. U.S. Pat. Nos. 2,334,756 and 4,313,247 describe several configurations for such cables, and methods and apparatus for the manufacture thereof
The insulation employed in mineral-insulated cables generally consists of a compacted mineral powder or other particulate, such as powdered magnesium oxide (MgO) or powdered aluminum oxide (Al2O3). As particulates, these insulation materials are subject to contamination by liquids and vapors, as well as to physical breakdown under stress or mechanical vibration. For this reason filling, crimping or other measures are often employed to seal mineral-insulated cable terminations against loss or contamination of the insulation in use.
A number of different termination designs for protecting the integrity of these cables have been proposed. U.S. Pat. No. 3,839,623 discloses the use of a ceramic end plug for this purpose, while U.S. Pat. No. 4,943,831 discloses a seal for a mineral-insulated thermocouple or thermistor which uses a crimped Teflon.RTM. polymer or mica plug to contain the insulation. U.S. Pat. Nos. 4,866,840, 5,149,916 and 5,221,709 disclose sealing structures comprising organic materials for relatively low temperature service.
A recent application for mineral-insulated cables is in the automotive field, where such cables have been proposed for use as power leads for supplying electrical energy to exhaust gas preheaters. Such preheaters, employed in the art as components of electrically heated catalytic converters, are used to preheat cold exhaust gas in order to accelerate the light-off of the catalysts which are used to remove carbon dioxide, hydrocarbons, and nitrogen oxide from the exhaust. Examples of converters of this type are disclosed in U.S. Pat. Nos. 5,238,650 and 5,271,906; co-pending U.S. patent application Ser. No. 08/347,723 entitled "Electrical Leads for a Fluid Heater", filed Dec. 1, 1994 by G. D. Lipp et. al. and commonly assigned herewith, discloses the use of mineral-insulated cables to replace the stud power leads of the earlier converter designs.
As disclosed in the aforementioned co-pending patent application, exhaust gases passing through an electrically heated catalytic converter may reach temperatures in the range of 800.degree.-1000.degree. C. in the course of engine operation. Heat from these gases is readily transmitted from the heater core through the electrical lead and power feedthrough to the electrical terminal outside of the heater enclosure. The mineral-insulated cables described in the copending patent application are effective to significantly reduce electrical terminal temperatures. Unfortunately, however, unless adequately sealed, the mineral insulation within the cables is subject to erosion and/or contamination through exposure to the moist, hot exhaust gases.
Durable mechanical sealing of the terminations of these cables is particularly difficult in the vibrationally severe environment of an automotive exhaust system. Both vibration and the extreme thermal cycling of the cable termination (from below-freezing temperatures to as high as 1000.degree. C.) place demands on the termination seal which cannot be met by conventional sealing materials or designs. In particular, known seals cannot provide the terminal durability necessary to provide the 50,000-100,000 mile service life presently required of automotive engine emissions control equipment.
Among the difficulties giving rise to present limitations on seal integrity are the differences in thermal and chemical properties between the metals used for cable sheathing and cable conductors. These differences give rise to a fundamental bonding incompatibility between the metals, such that refractory sealing materials customarily used for forming high-temperature-capable hermetic seals can form acceptable seals with one or the other of the metals, but not with both.
It is therefore a principal object of the present invention to provide a seal design, and a sealed termination for a mineral-insulated cable, which offer substantial improvements in the physical integrity and resistance to gas penetration of the seal.
It is a further object of the invention to provide a method for terminating a mineral-insulated cable lead which provides a hermetic seal against insulation loss or the contamination of the insulation by liquids or gases present in the surrounding environment.
Other objects and advantages of the invention will become apparent from the following description.